Combining ETD method and RLS algorithm in adaptive control, this paper presents a new algorithm named RLS&ETD for ultra-short term load forecasting of power systems. The dynamic tracking characteristic of adaptive control and the periodicity of ETD method are adopted in this method. The data are used more sufficient in the new method than that in ETD method. Finally, simulation results show that precision and stability are higher by RLS&ETD than that by ETD method.